1. Field of the Invention
The present invention relates to a drive circuit and drive method for a stepping motor for driving, e.g., a tube pump mounted to a medical device.
2. Description of the Related Art
There is conventionally known a tube pump as a pump device for use in a medical device. The tube pump is designed to convey a liquid within a liquid-circulating tube by rotating a rotor unit having a pressing roller with a drive motor and squeezing the tube with the pressing roller.
As one example of the tube pump of this kind, there is available a tube pump of the type in which a liquid is delivered little by little over a period of time. In the tube pump of this type, it is necessary to drive the rotor unit at a reduced speed. For that reason, the tube pump employs, e.g., a brushless motor, as a drive motor. The rotation speed of the motor is reduced by a speed reducing mechanism, thereby driving the rotor unit. To cope with the case that the motor becomes inoperable, the tube pump is provided with a manual operation mechanism for manually rotating the rotor unit.
Since a gear mechanism is often used as the speed reducing mechanism for reducing the rotation speed of the drive motor, it is unavoidable that noises are generated due to the meshing of the gears in the speed reducing mechanism. Thus, the conventional tube pump suffers from a problem of increased noises. In particular, an extremely high level of silence is required in a medical device. The noise problem is therefore of paramount importance in the tube pump for use in a medical device.
In the conventional tube pump, if the rotor unit is rotated by the manual operation mechanism, the rotor shaft of the motor is simultaneously rotated by way of the speed reducing mechanism. Thus, a high amount of force is required in the manual operation. For that reason, the conventional tube pump poses a problem in that the operability in the manual operation is very low.
In addition, since the tube is squeezed by the pressing roller in the conventional tube pump, a reaction force (load) of the tube continuously acts on the pressing roller. Therefore, if the motor is stopped to thereby halt the operation of the tube pump, the pressing roller is returned back, which may possibly cause the liquid to flow backward.
As a solution to these problems, it has been contemplated that a stepping motor could be used as the drive motor and the rotor unit is directly driven by the motor shaft with the speed reducing mechanism omitted. In this case, the omission of the speed reducing mechanism makes it possible to reduce the noises and to enhance the operability in the manual operation. In addition, the stepping motor is driven to maintain the stop position when stopped. Accordingly, it is possible to avoid occurrence of a phenomenon that the pressing roller is returned back by the reaction force of the tube, and to prevent the liquid from flowing backward.
Although the stepping motor, when stopped, can be maintained in the stop position by supplying stator coils with the same drive current as supplied during operation, there is a problem in that the stepping motor generates a larger amount of heat than that generated by a brushless motor. For that reason, if the stepping motor is applied to a tube pump, there appears a drawback that the temperature of the drive motor is increased to a level far higher than in the conventional case. In case of a medical device, when the properties of the liquid conveyed are taken into account, it is necessary to avoid a temperature increase in the liquid caused by the drive motor as much as possible. If the rotor unit of the tube pump is directly driven by the drive motor, the heat of the drive motor will be easily transferred to the rotor unit because the speed reducing mechanism is omitted. Accordingly, it is necessary to further reduce heat generation in the drive motor when the rotor unit is directly driven by the stepping motor.